Ultrasonic imaging from within the body of a patient has been used for some time, as for example as a preliminary step in the treatment of heart disease. While alternative methods of sensing the condition of diseased vasculature exist, such as the injection and monitoring of radiopaque dyes, ultrasound is currently the most promising technology for accurately viewing the interior of a body in real time and in a non-destructive manner.
Such imaging techniques are particularly useful in connection with an angioplasty device that removes a built-up deposit within a lumen. Successful removal depends upon accurately locating the deposit in relation to the device. It is desirable that the resolution of a visualization technique be commensurate with, or better than, the degree of resolution of the ablation device. The present invention has particular utility with high resolution ablation devices. An example of such a device is described in U.S. Pat. Nos. 5,626,576 and 5,454,809, commonly owned with the present invention. In such a device, radio frequency current is selectively deployed around the circumference of a lumen such as a coronary artery, depending upon the position of the occlusive material. The imaging used with such a device should enable the user to determine the circumferential position of the deposit. In general, prior art ablation techniques have not required the visualization resolution provided by the present invention.
In an in vivo ultrasound imaging system, either a rotating single transducer element or an array of piezoelectric transducer elements residing on a catheter are introduced into a body. The element(s) are excited at ultrasound frequencies to transmit acoustical waves, and receive echos as the acoustical waves reflect from the surrounding material. The echos provide electrical signals which are processed to form the ultrasound image.
Previous in vivo ultrasound images have included a number of limitations. The simplest systems use a mechanical scanning system. A flexible drive cable rotates a single element to scan a cross sectional image of a lumen. Problems associated with mechanical systems include mechanical breakdown, image jitter and smear, limitations due to fixed transmit and receive focus, and low image resolution away from the focal point. Existing electronic imaging systems address some of the above impediments but are still believed to have associated problems such as low signal to noise ratio and/or low frame rate (depending on whether multiple samples of element pulses are taken), reduced tissue penetration (because of relatively low transmit energy), narrow bandwidth pulses, and sub-optimal axial and lateral resolution (because only one matching layer is used). A modern electronic imaging system is described in international application Publication No. WO 97/23865 (Endosonics). The device of said application may serve its intended purpose, but does not disclose a technique to include complicated circuitry on a flexible circuit, and does not provide optimal acoustic matching or backing. A similar electronic imaging system is disclosed in European Patent Application No. 0 671 221 A2 (Intravascular Research Limited). The present invention effectively addresses the above problems through numerous design advances.
More complex electronic processing enables superior ultrasound imaging. Such processing requires a number of advances in the circuitry and associated components. In particular, incorporating a greater number of transducer elements is advantageous since it provides improved flexibility in manipulating a transmit and receive aperture, and element phasing. Also, the incorporation of advanced design IC chips improves signal to noise ratio and penetration and frame rate. The use of an extra matching layer improves signal bandwidth and axial resolution. Without limitation, it is an object of the present invention to satisfy the above mentioned imaging criteria and thereby advance the field of intravascular ultrasound imaging.
The transducer array probe described herein has particular application with the MUX/pre-amplifier ASIC described in co-pending application No. 08/950,900 titled "Intravascular Ultrasound Mixed Signal Multiplexer/Pre-Amplifier BiCMOS ASIC", filed under attorney docket no. 010848-0015 contemporaneously herewith. However, it is not limited to such a device, and is operable using other circuitry.
All documents referred to herein are hereby incorporated by reference to the extent they contain information necessary or helpful to an understanding of the present invention.